refactor(error): rename Kind to ErrorKind and update related error ha…

[yangfa-zhang]

feat(transport): expose ErrorKind for programmatic error discrimination (#2563)

The internal Kind enum that classifies transport::Error variants was
pub(crate), leaving tonic::transport::Error opaque to downstream users:
there was no way to programmatically distinguish a transport-level failure
from an invalid URI, an invalid user-agent header, or a TLS-on-UDS
misconfiguration without parsing Display output or downcasting source().

Changes:

• Rename Kind to ErrorKind and make it pub, with #[non_exhaustive]
  so future variants can be added without a breaking change.
• Derive Clone, Copy, Debug, PartialEq, Eq, Hash on ErrorKind (all
  variants are dataless), matching the style of CompressionEncoding.
• Add a public Error::kind(&self) -> ErrorKind accessor.
• Re-export ErrorKind from tonic::transport.
• Add unit tests covering each variant and the Copy bound.

Downstream consumers can now write:

  match err.kind() {
      ErrorKind::InvalidUri => /* ... */,
      ErrorKind::Transport => /* ... */,
      _ => /* required because #[non_exhaustive] */,
  }